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SCIENCE 



[N. S. Vol. XLIII. No. 1110 



thing that offered itself was an alteration in the 

 direction of the plumb-line with which the instru- 

 ment was constantly rectified; but this upon trial 

 proved insufficient. Then I considered what re- 

 fraction might do, but here also nothing satisfac- 

 tory occurred. At length I conjectured that all 

 the phenomena hitherto mentioned, proceeded from 

 the progressive motion of light and the earth's 

 annual motion in its orbit. For I perceived that, 

 if light was propagated in time, the apparent place 

 of a fixed object would not be the same when the 

 eye is at rest, as when it is moving in any other 

 direction than that of the line passing through the 

 eye and the object; and that, when the eye is 

 moving in different directions, the apparent place 

 of the object would be different." 



When Bradley's observations of y Dra- 

 eonis were corrected for aberration, they 

 showed, according to himself, that the par- 

 allax of that star could not be as much as 

 1".0, or that the star was more than 200,- 

 000 times as distant from the earth as the 

 sun. 



On December 6, 1781, there was read be- 

 fore the Royal Society a paper by Mr. 

 Herschel, afterwards Sir William, on the 

 "Parallax of the Fixed Stars." We read: 



The method pointed out by Gralileo, and first at- 

 tempted by Hook, Flamstead, Molineaux and 

 Bradley, of taking distances of stars from the 

 zenith that pass very near it, though it failed with 

 regard to parallax, has been productive of the 

 most noble discoveries of another nature. At the 

 same time it has given us a much juster idea of 

 the immense distance of the stars, and furnished 

 us with an approximation to the knowledge of 

 their parallax that is much nearer the truth than 

 we ever had before. . . . 



In general, the method of zenith distances la- 

 bours under the following considerable difficulties. 

 In the first place, aU these distances, though they 

 should not exceed a few degrees, are liable to re- 

 fractions; and I hope to be pardoned when I say 

 that the real quantities of these refractions, and 

 their differences, are very far from being perfectly 

 known. Secondly, the change of position of the 

 earth's axis arising from nutation, precession of 

 the equinoxes, and other causes, is so far from 

 being completely settled, that it would not be very 

 easy to say what it exactly is at any given time. 

 In the third place, the aberration of light, though 



best known of all, may also be liable to some small 

 errors, since the observations from which it was 

 deduced laboured under all the foregoing difficul- 

 ties. I do not mean to say, that our theories of 

 all these causes of error are defective; on the con- 

 trary, I grant that we are for most astronomical 

 purposes sufficiently furnished with excellent tables 

 to correct our observations from the above men- 

 tioned errors. But when we are upon so delicate 

 a point as the parallax of the stars; when we are 

 investigating angles that may, perhaps, not amount 

 to a single second, we must endeavor to keep clear 

 of every possibility of being involved in uncer- 

 tainties; even the hundredth part of a second be- 

 comes a quantity to be taken into consideration. 



Herschel then proceeds to advocate se- 

 lecting pairs of stars of very unequal mag- 

 nitude and whose distance apart is less than 

 five seconds, and making very accurate 

 micrometric measures of this distance from 

 time to time. The first condition should 

 give, in general, stars very unequally dis- 

 tant from the earth, so that the changing 

 perspective as the earth revolves in her 

 orbit would give a variation of the appar- 

 ent distance between the stars, while the 

 small distance, less than five seconds, would 

 eliminate from consideration entirely any 

 effect upon this distance of the uncer- 

 tainties in refraction, precession, nutation, 

 aberration, etc. Herschel had already com- 

 menced the cataloguing of such double stars 

 and in January, 1782, submitted to the 

 Royal Society a catalogue of 269. This 

 work did not enable Herschel to determine 

 the distance of the stars but did enable 

 him to demonstrate that there exist pairs 

 of stars in which the two components re- 

 volve the one around the other. In twenty 

 years he had found fifty such pairs. 



Coming forward another generation, that 

 is, to a time a little less than a hundred 

 years ago, we find Pond, then Astronomer 

 Royal, writing 



The history of annual parallax appears to me to 

 be this: in proportion as instruments have been 

 imperfect in their construction, they have misled 

 observers into the belief of the existence of sen- 



